(1) Field of the Invention
This invention relates to a method and apparatus for wind speed measurements using a laser radar (lidar) wind speed measurement system. More particularly, the invention relates to wind speed measurement apparatus for use on floating platforms such as buoys.
(2) Description of the Art
At present, there is much interest in the exploitation of renewable energy resources in an attempt to reduce the greenhouse gases produced by burning fossil fuels. One type of renewable energy resource that has received much attention is wind power.
Land based wind farms comprising a number of wind turbines have been used commercially to produce energy for many years. However, finding sites that are suitable for such wind farms has proved problematic, especially in the light of local environmental objections. Recently, this has led to the development of off-shore wind farms where the environmental impact is greatly reduced. Furthermore, such wind farms are able to exploit the higher wind speeds that are typically found at sea.
Selecting suitable off-shore sites for wind turbine placement is particularly important to ensure the energy generated by the turbine is sufficient to offset the relatively high construction costs. However, the process of determining suitable off-shore sites has been hindered by a number of problems that are not encountered when assessing the suitability of land based sites. For example, prior to the siting of a wind turbine on land it is typical to log the wind speed at the proposed site for a prolonged period of time (e. g. twelve months or more) in order to ensure the wind regime is suitable. Such wind speed measurements, which are preferably made at the height above the ground at which the turbine blades will be located, are typically performed on land by erecting a mast that carries a suitable mechanical or sonic anemometer. Similar types of measurement have proved difficult to make off-shore.
It has previously been attempted to log off-shore wind speed data using mast mounted mechanical or sonic anemometers analogous to those used on land. The masts have been directly fixed to the sea bed or mounted on buoyant platforms such as barges or buoys. As the blades of off-shore wind turbines are typically located many tens (possibly hundreds) of meters above the surface of the water the mast should, ideally, be sufficiently tall to locate the conventional anemometer in a similar position. However, the cost of forming the necessary foundations in the sea bed to directly support a mast can be prohibitive. Similarly, it has only been possible to make periodic measurements using manned barges because mooring a barge at a potential site for a long period of time is simply too costly. Furthermore, the construction of buoys sufficiently stable for carrying the relatively tall mast and conventional anemometer arrangement has proved technically challenging.
More details about some of the problems associated with wind speed data collection at sites located offshore are given by Grainger, W. , Gammidge, A, and Smith, D. , in the paper entitled “Offshore wind data for wind farms” published in the proceedings of the twentieth British Wind Energy Association wind energy conference (“wind energy—switch to wind power”), ISBN-1-86058-374-4.
In addition to mast mounted conventional anemometry systems, ground based lidar systems are known. Lidar systems provide wind speed data by measuring the Doppler shift imparted to laser light that is scattered from natural aerosols (e. g. dust, pollen, water droplets etc. ) present in air. An example of a CO2 laser based lidar system is described by Vaughan and Forrester in Wind Engineering, Vol 13, No 1, 1989, pp 1-15; see in particular section 8 thereof. More recently, lower cost optical fibre based lidar devices of the type described in Karlsson et al, Applied Optics, Vol. 39, No. 21, 20 Jul. 2000 have been developed.
Lidar systems measure the Doppler shift imparted to reflected radiation within a certain remote probe volume and can thus only acquire wind velocity data in a direction parallel to the transmitted/returned laser beam. In the case of a lidar device located on the ground, it is possible to measure the true (3D) wind velocity vector a given distance above the ground by scanning the lidar in a controlled manner; for example using a conical scan. This enables the wind vector to be intersected at a range of known angles thereby allowing the true wind velocity vector to be constructed. Ground based scanned lidar systems have been used to measure wind sheer, turbulence and wake vortices for many years in both military and civil applications; for example see Laser Doppler Velocimetry Applied to the measurement of Local and Global Wind, J. M. Vaughan and P. A. Forrester, Wind Engineering, Vol. 13, No. 1, 1989.